1. Field of the Invention
The present invention relates to an illumination optical system which is suitable for anneal processing of a glass substrate and the like, and a laser processor incorporating the optical system.
2. Description of the Related Art
One conventional crystallizing technique is performed by radiating laser light onto an amorphous silicon film. There is another technique of radiating laser light in order to repair the crystal properties of the silicon film, which is damaged by the injection of impurity ions, and to revitalize injected impurity ions. This is called a laser anneal method.
One characteristics of laser anneal processing is that there is almost no heat damage to the substrate. This characteristic of causing no heat damage to the substrate is useful when, for example, providing a semiconductor element on a substrate having low heat-resistance, such as glass.
Recently, a glass substrate is preferably used as the substrate in a liquid crystal display, and in particular in a liquid crystal display for moving images, in view of cost concerns and the demand for a larger area. When using the laser anneal method, even when the substrate comprises a glass having low heat-resistance, there is almost no heat damage to the glass substrate. Therefore, it is possible to construct a semiconductor element such as a thin-film transistor comprising a crystallized silicon film, even when using a glass substrate. Consequently, in the future, the laser anneal method is expected to be an important technological feature in the construction of semiconductor circuits on glass substrates.
Most glass substrates with a semiconductor circuit or the like mounted thereon have a comparatively large area. In contrast, the beam radiation area of laser light immediately after emission from the light source is small. For this reason, the shape of the beam is made square or linear, and a predetermined region is scanned. For example, a linear beam of light is moved perpendicular to its long side and scans the glass substrate. By this method, anneal processing of the entire glass substrate can be completed in a comparatively short time.
An optical system for making a linear beam used in such laser anneal processing is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. 10-244392. In Japanese Unexamined Patent Application, First Publication No. 10-244392, a laser beam is made linear by using an optical system known as a homogenizer. The homogenizer is required to make a linear beam having extremely uniform illumination and shape. In the above publication, a multi-cylindrical lens system comprising a plurality of cylindrical lenses is used as the homogenizer. The homogenizer plays a central role in ensuring that the illumination of the beam is uniform.
In the multi-cylindrical lens system, strip-like cylindrical lenses are arranged in a row parallel to the direction of their refractive power. In the same manner as a fly-eye lens used in conventional uniform illumination, a bundle of rays, radiated into the multi-cylindrical lens system, is split by the cylindrical lenses and focused into a linear shape. As a result, the number of linear images is the same as the number of cylindrical lenses.
These linear images become a plurality of new secondary light sources, which radiate light onto a sample via another cylindrical lens. The lights from the plurality of secondary light sources reconnect on the illuminated face of the sample, and are averaged. Therefore, the illumination distribution becomes uniform in the direction which the multi-cylindrical lens system is arranged in (the direction having refractive power).
Furthermore, in Japanese Unexamined Patent Application, First Publication No. 10-244392, two multi-cylindrical lens systems are used to make the illumination uniform in the direction parallel to the width of the linear beam as well as parallel to the length of the beam.
However, the following problems arise when using a large number of cylindrical lenses, as described in the above publication. Making a cylindrical lens is not as easy as a normal spherical-faced lens, and the manufacturing cost is higher. Furthermore, the shaping precision is greatly inferior to that of the normal spherical-faced lens. Considering the manufacture of an actual apparatus, there is a possibility that an optical system comprising a great number of cylindrical lenses will increase the manufacturing cost and fail to satisfy demands for high work precision.
The growing demand for large-scale liquid crystal displays has been accompanied by an increase in the area of the scanning region. Consequently, there is a demand for an even longer linear beam. When the length of the linear beam is increased while keeping its width constant, the illuminated area increases. Therefore, the energy density per area unit decreases. As a result, when the beam is radiated onto a sample, it is difficult to heat the sample to the temperature required for anneal processing. Accordingly, to increase the energy density when illuminating the sample, the long-side length of the beam need increase, also the width of the beam need be narrower.
Other reasons why a beam with a narrow width is needed are explained as follows. An excimer laser has high output power, and is widely used as a laser light source. However, the excimer laser is expensive, and the apparatus itself is large. For this reason, it is desirable to use a fixed laser, or a YAG laser, which is cheaper, smaller, and easier to handle, as the laser light source. The fixed laser and YAG laser have lower output energy than the excimer laser. Therefore, in order to increase the energy density on the illuminated face, the light must be focused to form a narrower beam. Consequently, not only the length of the beam need increase,also its width need be narrower.
As described above, the need for a linear beam with a narrow width requires an optical system with high image formation capability in the direction of the length of the linear beam. In view of the demand for this type of image formation capability, the specifications of the optical system disclosed in Japanese Patent Laid-Open No. 10-244392, mentioned above, are inadequate.
As described above, Japanese Unexamined Patent Application, First Publication No. 10-244392 uses two multi-cylindrical lens systems having a plurality of strip-like cylindrical lenses. The optical system following the multi-cylindrical lens systems is generally called a condenser lens, and similarly comprises a multi-cylindrical lens system.
Constructing the optical system using a group of cylindrical lenses in this way, and constructing the optical system from power apparatuses having different beam long-side directions and short-side directions, is believed to be an effective design method, making the apparatus easier for a designer to comprehend intuitively when creating a rectangular (linear) beam,
However, in an optical system which combines cylindrical lenses with different directions of power, when a bundle of parallel rays enters, there is a light beam which travels in a different direction to the directions of power of the cylindrical lenses. The aberration of this light beam cannot easily be corrected by an optical system which simply combines power in intersecting directions. Therefore, this design method is not desirable when attempting to correct the aberration of the optical system to a high level.
For example, that the bundle of parallel light rays is assumed to be circular in cross-section. Then, a first cylindrical lens having negative (concave) power is provided, and a second cylindrical lens having positive (convex) power in a direction intersecting the power direction of the first cylindrical lens, is provided behind the first cylindrical lens (on the image side). Then it is assumed that the parallel light rays enter the first and second cylindrical lenses, and are focused into a linear shape.
In this case, the first cylindrical lens which has negative power disperses the light rays in one direction only. Then, the subsequent second cylindrical lens which has positive power focuses the dispersed light in a direction perpendicular to the direction of the dispersion. The light at the center of the dispersion, emitted from the negative first cylindrical lens, enters the positive second cylindrical lens at a perpendicular to the mother line of the second cylindrical lens. On the other hand, the light at the peripheral section of the dispersion, emitted from the first cylindrical lens, enters the second cylindrical lens at a diagonal to the mother line of the second cylindrical lens.
As a result, the central and peripheral light, emitted from the negative first cylindrical lens, have different focal positions after entering the positive second cylindrical lens. Consequently, when the image is formed in a linear shape, the width of the line at the center of the linear image is different from that at the periphery. Therefore, in an optical system comprising a cylindrical lens, this characteristic aberration of the cylindrical lens must be corrected.
Generally, optical designers are not familiar with the characteristic aberration of cylindrical lenses described above. The behavior of the light beam cannot be expressed simply in terms of one face in the short (short axis) direction of the beam and another face in the long (long axis) direction. It is extremely difficult to correct the characteristic aberration of the cylindrical lens with only a combination of cylindrical lenses with intersecting powers. Even if the aberration were to be corrected, the large number of cylindrical lenses required would be enormous.
As described above, when making a linear beam with a narrow width, an optical system which uses a large number of intersecting cylindrical lenses is not desirable from an optical design point of view.
The optical system disclosed in Japanese Unexamined Patent Application, First Publication No. 10-244392 has a constitution which maintains uniform illumination in the direction of the short side of the beam. However, this structure is not desirable for working a linear beam with a narrow width, for the following reasons.
Firstly, the need for uniform illumination along the width of the line will be explained. Increasing the illumination uniformity along the width of the line is effective when increasing the scanning speed of the linear beam. When the width in the scanning direction of the linear beam is wide, no matter how fast the scanning speed of the linear beam is , the total time for the linear beam to pass the unit area on the sample substrate and consequently becomes a long. The illumination time of the linear beam on the sample substrate is sufficient for a reaction such as crystallization to take place. Therefore, a wider linear beam enables the scanning speed to be increased, the anneal time can be shortened.
However, when the linear beam has poor illumination uniformity, energy decreases at the peripheral portions of the beam width. Consequently, when scanning the linear beam, there is no anneal reaction at the peripheral portions of the beam. This is equivalent to scanning a linear beam with a narrow width, and makes it impossible to increase the scanning speed.
As already mentioned, there is a recent demand for liquid crystal displays with a large area. Therefore, techniques for making large-area substrates are desirable in order to increase the manufacturing speed of the liquid crystal display. As described above, aberration should be corrected to a high degree in order to reduce the width of the linear beam. It is extremely difficult to make a linear beam with a narrow width after aberration has been corrected to a high degree, while maintaining highly uniform illumination in the short (width) direction of the linear beam. Therefore, there is a demand for an optical system which is specially designed to obtain highly uniform illumination in the long direction of the linear beam, and to reduce the width in the short direction of the linear beam. For this point of view, the optical system disclosed in Japanese Unexamined Patent Application, First Publication No. 10-244392 cannot be regarded as adequate.
Furthermore, in an optical system which emits laser light over an extremely large scanning range, it is very difficult to increase the number of apertures (NA) in the optical system on the emission side. This causes diffraction, so that optical considerations alone are not sufficient to analyze the uniform illumination of the linear beam. Moreover, the optical system mentioned above uses a multi-cylindrical lens system to increase the illumination uniformity in the short direction of the linear beam. As already mentioned, the function of this lens system is to split the beam emitted from a light source in the direction of its width, and reconnect linear images, formed by the split beams, on an illuminated face. Therefore, when the width of the linear images of the illuminated face become narrow, the reconnection precision of the linear images must be made smaller than the linear image width. In other words, the precise reconnection of the linear image becomes more difficult as the required line width becomes narrower. Therefore, when consideration is also given to the manufacture of the laser processor, even when some of the uniformity of the illumination distribution of the linear image on the illuminated face can be sacrificed, it is still preferable to reduce the number of split beams. By slightly reducing the speed of the annealing work, the illumination uniformity of the linear image parallel to its width can be reduced and the number of split beams can be reduced; thus, this is desirable with regard to manufacturing the apparatus.
The present invention has been realized in consideration of the problems described above. It is an object of this invention to provide an illumination optical system which has superior image formation capabilities, and can radiate a linear beam with excellent illumination uniformity and a narrow line width having a large aspect ratio. It is another object of this invention to provide an inexpensive, easily manufactured laser processor which can process a large area at high speed.